1. Field of the Invention
The present invention relates to a radio communication system, and in particular to a method for determining sizes of packet data units when control packet data units are transmitted at the same time in an E-UTRA communications system.
2. Description of the Prior Art
The following description contains many terms related to the E-UTRA communications system. For simplicity of explanation, the following acronyms will be used.
AM=Acknowledged Mode
MAC=Medium Access Control
PDU=Packet Data Unit
RLC=Radio Link Control
SDU=Service Data Unit
TB=Transport Block
TP=Transmission Payload
TM=Transparent Mode
UE=User Equipment
UM=Unacknowledged Mode
UMTS=Universal Mobile Telecommunication System
Radio Link Control (RLC) is a protocol for a user equipment-Evolved UMTS Terrestrial Radio Access Network (UE-EUTRAN) radio interface. Communications packets are transmitted between a physical layer, a MAC layer, and an RLC layer. The RLC layer consists of three RLC entities: a TM entity, a UM entity, and an AM entity. Amongst these entities, the AM entity exchanges status information using Control PDUs to guarantee reliable transmissions. This system of receiving transmission acknowledgments enables a transmitting AM entity to retransmit RLC PDUs that are not received by a receiving AM entity. The AM entity is therefore bi-directional
An AM entity includes a transmitting side and a receiving side. The MAC layer selects a TB size in a next transmission timing. In the transmitting side, RLC SDUs are segmented and/or concatenated into AM Data PDUs of a variable length at a segmentation/concatenation unit according to the TB size selected by MAC. If Data PDUs to be retransmitted do not fit into the new TB size selected by MAC at the particular transmission opportunity notified by MAC, the AM entity segments the retransmission PDUs into retransmission PDU segments. The number of times re-segmentation can occur is not limited. The Data PDU may therefore contain segmented and/or concatenated RLC SDUs (depending on the size of the PDU). Length indicators are used to define boundaries between RLC SDUs within Data PDUs.
The AM entity can request a status report from the peer AM entity to confirm whether transmitted Data PDUs are received, by setting a polling bit in the Data PDUs. The receiving side of the peer AM entity will then generate Control PDUs, where Data PDUs buffered in the Retransmission buffer are deleted or retransmitted based on a status report found within the Control PDUs.
The PDUs are delivered to a unit that completes the Data PDU header based on an input from an RLC Control Unit that indicates values to set in various fields (e.g. Polling Bit). Once a complete RLC SDU has been received in the peer AM entity, the associated Data PDUs are reassembled by a Reassembly Unit and delivered to upper layers.
In the current E-UTRA RLC spec [1], the RLC SDUs are segmented and/or concatenated in accordance with the TB size selected by MAC at the particular transmission opportunity notified by MAC. When a Control PDU is also transmitted or two or more RLC entities with equal logical channel priority have data to transmit, the segmentation/or concatenation according to TB size selected by MAC may not be suitable. This situation is illustrated by the following examples.